The preparation of carboxylic acids by contacting olefins with carbon monoxide normally requires the use of hazardous and corrosive compounds such as alkyl halides or strong acids, such as hydrogen halides or sulfonic acids (commonly referred to as co-catalysts and/or promoters) or extreme hydrocarboxylation conditions, i.e., extreme pressures and temperatures. Historically, the direct hydrocarboxylation of olefins to carboxylic acids, a process referred to commonly as hydrocarboxylation, entails the use of significant quantities of hazardous and corrosive materials such as alkyl halides (which generate hydrogen halides in situ) or strong acids such as hydrogen halides or sulfonic acids. Such materials commonly are referred to as co-catalysts or promoters. Extreme process pressures and temperatures have been employed in the absence of the corrosive materials mentioned above. Numerous examples of processes utilizing an alkyl halide or strong acid are known in the prior art and are discussed by J. R. Zoeller, U.S. Pat. Nos. 5,760,284; 5,936,117; and 5,977,407 as well as by W. Bertloff, Carbonylation, Ulmann's Encyclopedia of Industrial Chemistry, 6th Edition, Vol. 6, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, page 473 (2003); and W. Rienmenschneider, “Carboxylic Acids, Aliphatic”, Ulmann's Encyclopedia of Industrial Chemistry, 6th Edition, Vol. 6, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, page 493 (2003). U.S. Pat. No. 6,916,951-B2 and A. Riisager, et. al., Chemical Communications, pages. 994-996 (2006) disclose carbonylation processes conducted in the presence of an onium salt and a hydrocarboxylation catalyst wherein gaseous halide such as gaseous methyl iodide is added continuously to a reaction zone.
To avoid the use of a strong acid or alkyl halide in hydrocarboxylation processes for manufacturing carboxylic acids, the use of extreme pressures and temperature or alternative chemistry is necessary. One example of a high pressure and temperature system that does not employ a strong acid is the nickel-catalyzed carbonylation of ethylene to propionic acid that is operated on a commercial scale. This nickel-catalyzed process involves the use of highly toxic nickel carbonyl as the active catalyst at temperatures greater than 270° C. and pressures greater than 186 bar gauge (barg; 2700 pounds per square inch gauge—psig) in a silver lined reactor. This nickel-catalyzed process is described by W. Bertloff, Carbonylation, and U.-F. Samel, W. Kohler, A. O. Gamer, and U. Keuser, Propionic Acid and Derivatives Ulmann's Encyclopedia of Industrial Chemistry, 6th Edition, Vol. 30, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, p. 261 (2003). Descriptions of other, non-commercial processes may be found in (i) Schafer, Hohn, and Lippert, U.S. Pat. No. 5,866,716, which describes a rhodium/nitrogen heterocycle, e.g., pyridine, catalyst system that, while operating at more moderate temperatures, e.g., 100° C., operates at high pressure, e.g., approximately 100 barg (1470 psig). The process disclosed in U.S. Pat. No. 5,866,716 has some separation difficulties associated with removing the nitrogen heterocycle and produces diethyl ketone by-product. Lippert, Hohn, Schafer, and Hupfer, U.S. Pat. No. 5,705,683 describes an improved nickel-catalyst process that operates at high pressure, e.g., 100 barg, 1470 psig) and temperature, e.g., 200° C. Such high pressures require specialized and costly equipment. These processes also have the disadvantage of producing some diethyl ketone by-product which complicates separation and product purification.
Hydrocarboxylation processes such as, for example, the conversion of ethylene to propionic acid, typically require hydrogen iodide and/or alkyl iodide (e.g., ethyl iodide) to be fed to the reaction zone wherein the hydrocarboxylation reaction occurs. The feed of hydrogen iodide and/or alkyl iodide is problematic since the hydrogen iodide and/or alkyl iodide are corrosive, must be removed from the product and recycled in subsequent distillation steps and, due to its toxicity and volatility, requires very rigorous and expensive process controls. Elimination of the requirement to add significant volumes of alkyl iodide would reduce significantly the costs associated with separation and the expensive control equipment associated with safely handling such a volatile and toxic component.